stage2: implement basic unions

 * AIR instructions struct_field_ptr and related functions now are also
   emitted by the frontend for unions. Backends must inspect the type
   of the pointer operand to lower the instructions correctly.
   - These will be renamed to `agg_field_ptr` (short for "aggregate") in
     the future.
 * Introduce the new `set_union_tag` AIR instruction.
 * Introduce `Module.EnumNumbered` and associated `Type` methods. This
   is for enums which have no decls, but do have the possibility of
   overriding the integer tag type and tag values.
 * Sema: Implement support for union tag types in both the
   auto-generated and explicitly-provided cases, as well as explicitly
   provided enum tag values in union declarations.
 * LLVM backend: implement lowering union types, union field pointer
   instructions, and the new `set_union_tag` instruction.

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